1. Description Of The Prior Art
This invention relates to apparatus for supporting the tubes of a nuclear steam generator to inhibit vibration and, more particularly, to such apparatus that permits the ready insertion of the tubes, while providing improved support and extended life of the tubes.
2. Reference to Related Application
Cross reference is made to co-pending application Ser. No. 490,060 now U.S. Pat. No. 4,570,883, filed on Apr. 29, 1983 in the name of this inventor and entitled TUBE SUPPORT GRID.
2. Description Of The Prior Art
A nuclear steam generator 10 of the type found in the prior art is shown in FIG. 1 of the attached drawings, as comprising a bundle of a large number of vertically orientated U-shaped tubes 30. The tubes 30 are disposed in a lower, cylindrically shaped shell 12 of the steam generator 10, whose bottom end is associated with a primary coolant plenum or channel head 16, typically of a hemi-spherical configuration as shown in FIG. 1. The channel head 16 bis divided by a partition 22 into a first half typically known as a hot leg 18, and a second half typically known as a cold leg 20. High-temperature primary water from a nuclear reactor is introduced into the steam generator 10, through a primary inlet 24 into the hot leg 18. The primary water passes from the hot leg 18 into the exposed openings of the plurality of U-shaped tubes 30, flows through the tubes 30 to be introduced into the cold leg 20, and finally, exits from the steam generator 10 through a primary outlet 26.
Opposite ends of the U-shaped tubes 30 are mounted within a tube sheet 28 in communication with the hot leg 18 and the cold leg 20, respectively. A wrapper 14 surrounds the bundle of U-shaped tubes 30. In FIG. 1, a part of the wrapper 14 is broken away to show one of a plurality of tube support plates 32. It is understood that the other tube support plates 32 are disposed above the illustrated plate and in parallel relationship thereto to support the bundle of tubes 30 in a manner to inhibit vibration induced by the secondary water circulating through the steam generator 10.
As shown in FIG. 1, the tube support plate 32 has a plurality of tube holes 34 disposed therethrough, each for receiving a tube 30. The secondary water circulating within the steam generator 10 and, more particularly, faulted conditions of feedline and steamline breaks imposes a load on the tube support plate 32. The load handling capability of the tube support plate 32 is dependent upon its thickness and, in turn, upon the area of that section taken between the tube openings 34. Tube support plates of the prior art have been thickened to increase their load handling capacity. However, the ease of inserting the tubes 30 through their corresponding openings 34 is related to the thickness of the tube support plate 32. As the thickness of tube support plate 32 increases, the ease of inserting the tubes 32 therethrough decreases, thus necessitating an increase in the diameter of the tube holes 34 and, in particular, the diametral clearances between the tube holes 34 and the tubes 30. However, increasing the diametral clearances has a direct, negative effect on tube vibration and the expected life of the tubes 30. If the tubes 30 are permitted to vibrate with respect to their tube holes 34, their exterior surfaces will begin to wear and scar. Eventually, tube vibration will wear through the walls of the tubes 30 causing the primary water circulating therethrough to leak into the steam generator 10, eventually escaping to the turbine generator (not shown).
Tube support plates of the prior art are also subject to dry out or vapor blanketing at the area of contact between the tubes 30 and the tube support plate 32. Contaminants in the secondary water tend to collect at the contact areas between the tubes 30 and the tube support plate 32. The contaminants build-up at the contact areas tending to corrode and to weaken the tube walls. The combination of tube corrosion and vibration eventually leads to tube leakage.
The contact areas between the tubes 30 and the tube support plate 32 tends to be hot with respect to the surrounding environment, noting the direct contact of the support plate 32 with the tubes and the high temperature primary water passing therethrough. The secondary water circulating in the steam generator 10 tends to dissipate this heat if it is permitted to flow directly about the contact areas. However, fine particles of magnetite formed at the relatively high temperatures within the circulating secondary water tends to collect and build-up sludge patches about the tube openings 34 and, in particular, the contact areas 42, thus preventing the secondary water direct access to the contact areas and the dissipation of heat therefrom. As the sludge patches build-up, the vaporization occurring at the contact areas is not washed away by the circulating secondary water, thus, leading to dry out and corrosion of the contact areas. It is desired to decrease the sludge patches, whereby the distance between the center of the sludge patch to the wetted regions of the tubes is shortened. Removal of the sludge patches will increase heat transfer in the contact areas 42 and lower the available superheat at the center of the contact areas between the tubes 30 and the tube support plate 32. By lowering the temperature of the contact area and preventing vapor blanketing, corrosion of the contact area may be inhibited and the tube life extended.